A number of applications in microelectronics lithography, optics, and other fields benefit from highly accurate surface characterization of a substrate with respect to flatness, edge roll-off, uniformity, axial runout, and other dimensional features. There is particular interest in surface dimensional characterization, to sub-micron accuracy, in applications such as semiconductor wafer preparation and processing.
A familiar metric to those skilled in semiconductor wafer preparation and handling relates to a measurement called Roll-Off Amount (ROA), or referred to as linear roll-off amount (L-ROA), that relates to the edges of the wafer. Successful lithographic processing of the semiconductor wafer can be highly dependent on aspects of the mechanical profile near the edge of the polished wafer. Wafer manufacturers endeavor to accurately characterize and carefully control the wafer profile at the edge in order to meet stringent customer requirements. Industry-accepted ROA standards of measurement are defined, for example, by Semiconductor Equipment and Materials International (SEMI) as SEMI standard M69-0307.
The ROA measurement profiles the flatness characteristic in the region near the edge of the wafer and helps to identify flatness problems caused by polishing errors. The conventional measurement for ROA is typically performed using a stylus or a single point optical probe. The edge of the generally circular wafer is measured at eight different radial angles, typically every 45 degrees around the circumference, in a time-consuming and error-fraught process that can require costly equipment and highly trained technicians.
Interferometric techniques have been used to address the problem of surface edge profile characterization, but with somewhat disappointing results. For example, applying conventional phase-difference interferometric approaches requires dedicated measurement apparatus that holds the sample and reference surfaces in a highly rigid relationship and is well-buffered from vibration. Typical phase measurement algorithms acquire a number of interferograms, with precise equipment adjustment for change of phase between each image acquisition, and with this process repeated at multiple angular increments along the wafer edges. Given the number of steps required, the demanding requirements for precision and vibration protection for the measurement system and environment, and overall time that is needed, it can be appreciated that there is room for improvement in surface characterization methods, particularly methods better suited for edge profile characterization techniques for semiconductor wafers and highly flat substrate surfaces.